Currently CuproBraze™, or CuZnFe alloy heat exchangers use a brazed tube-to-header type joint. This joint, while being relatively strong, is prone to leaks after the initial brazing of the core if the process is not under precise control. Many variables can lead to leaks developing at the joint. These variables include poor tolerances in the header hole or tube geometry, poor paste application on the tube-to-header joint, poor heat profiles during brazing, as well as other factors.
The brazed tube-to-header joint is also prone to premature failure. The tube-to-header assemblies of Serpentine style radiators utilizing oblong tubes use a header with oblong openings that are typically the same shape as the tube, only slightly larger. The tube is bonded, non-mechanically, to this header using a brazing process. Such tube ends with an oblong cross-sectional shape will have a diameter in one direction greater than the diameter in another (usually perpendicular) direction, which is referred to herein as the “major diameter” and “minor diameter”, respectively.
Creation of a tube-to-header assembly or joint is accomplished by affixing a plurality of tubes having oblong ends into a plurality of corresponding oblong openings of approximately equal cross section in the header. As shown in the prior art (e.g., U.S. Pat. No. 5,150,520 to DiRisi), the tubes are inserted into corresponding openings in the header wall whereupon the minor diameter of the tube end is reduced and the major diameter of the tube end is increased to create a contacting fit around the circumference of the header.
Each tube is non-mechanically bonded to a corresponding collar opening in the header wall to form a plurality of tube-to-header joints. The collar openings are formed in the same operation when the plurality of openings are punched into the header.
Unfortunately, these prior art bonding processes add thermal stress to the tubes at their respective bonding locations, thereby increasing the grain size of the tube and reducing the tensile strength of the material at this point. A reduction in such tensile strength can and often times does result in pressure cycle fatigue and failure. This fatigue is also a result of the stresses applied during thermal cycling. Thermal cycling occurs during a cyclic change in coolant temperature, when idol coolant, initially at ambient temperature, becomes significantly hotter during use.
During the thermal cycle, deformation of the header may occur as a result of the weight of the heat exchanger and the coolants therein, thereby weakening the core-to-header assembly, which leads to failure of the bond. Furthermore, the addition of the secondary filler material, used to aid in strengthening the stressed tubes, can be a source for environmental concerns, such as the use of leaded solder for the secondary filler material.